A sound attenuator also called duct silencer is used to reduce noise in any heating, ventilation, and air conditioning system.
In a HVAC system, the potential noise sources are AHU, terminal units and room air outlets. The sound wave generated by these sources transfer its energy throughout the duct and to the occupant’s area as shown below.
Sound Power and Sound Pressure are different in that Sound Power is a measure of total energy per unit time emitted by the source in all directions. Sound pressure is a measure of pressure variation at the receiver’s location.
Sound Pressure is dependent on the acoustic environment. The factors involved include the effects of nearby reflecting surfaces, receiver distance, type of space, the amount of location in the space, the presence of barriers, and the intrusion of ambient sounds. Therefore, the sound pressure resulting from a given AHU — generated sound power depends on :
- Distance from the AHU to the room
- The size of the room
- The absorptive properties of interior furnishings
- Attenuating elements such as silencers, duct liner, duct branches, elbows , etc
Sound Power Output
AHU equipment supplier provides sound power data as given in the below table. The quietest sound we might measure is 10-12 Watts (0dB) and the loudest noise is that of space shuttle during take off which is 108 Watts (200 dB)
Insertion loss is the decrease in sound pressure levels that can be expected when a silencer is inserted into the path between the source and the receiver.
Pressure Drop is the difference in static pressure from the inlet to the outlet.
Regenerated Noise is the sound power created when airflows through a silencer at a given velocity and direction (forward or reverse).
Noise Criteria (NC) is a single number rating derived from sound pressure levels in all eight octave bands, and is intended to predict an occupant’s response to the overall sound level. The critical octave bands for evaluating sound performance ranges from 63 to 8000 Hz. To determine N C rating value, sound pressure levels are plotted with a family of criterion curves shown in Figure. The level which intersects the highest curves determines the overall NC rating.